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RESEARCH Open Access Cast accuracy obtained from different impression techniques at different implant angulations (in vitro study) Enas A. Elshenawy 1* , Ahmed M. Alam-Eldein 2 and Fadel A. Abd Elfatah 2 Abstract Background: Angulated implants may result in inaccurate impressions, and the impression technique may affect the accuracy of the definitive cast. This study was designed to compare the dimensional accuracy of casts obtained from three impression techniques for three definitive lower casts with implants at different angulations. Methods: Three Osseolink implants were placed in three reference models with different angles (parallel, 15° and 30°). Impressions of each model were made with three techniques (n = 10 per group): indirect, unsplinted direct, and acrylic resin-splinted direct technique. Impressions were poured with type IV dental stone. Inter-implant distances were measured for casts using a coordinate measuring machine, and the deviations from the reference models (Δr) were calculated. Data were analyzed using one-way ANOVA followed by post hoc tests to detect significance between groups (α = 0.05). Results: This study showed that the deviations in micrometers from the reference model were the least for acrylic resin-splinted direct technique (Δr1 = 49.96, Δr2 = 50.36) versus indirect (Δr1 = 93.8, Δr2 = 90.9) and unsplinted direct techniques ((Δr1 = 67.07, Δr2 = 68.66) in 30° angulated implant situation (p value < 0.0001 * for both Δr1 and Δr2). In 15° angulated implants, both the acrylic resin-splinted direct (Δr1 = 44.64, Δr2 = 45.58) and unsplinted direct techniques (Δr1 = 47.39, Δr2 = 55.28) were more accurate than indirect technique (Δr1 = 64.8, Δr2 = 68.3) (p value < 0.0001 * for both Δr1 and Δr2). While in parallel condition, no difference was found between all three techniques (p value = 0.085, 0.056 for Δr1 and Δr2, respectively). Conclusions: The impression technique affected the accuracy of definitive casts. The acrylic resin splinted direct technique produced the most accurate casts, followed by direct unsplinted and indirect techniques. Furthermore, implant angulation affected the impression accuracy. When implant angulation increased from parallel implants to 30°, the forces of deformation increased, which resulted in increased distortion. Keywords: Direct technique, Indirect technique, Internal connection implant, Splinting procedure Background Precise working casts are essential to fabricate passively fitting implant prostheses. Accurate implant impressions play a significant role and serve as a starting point in the process of producing good working casts [1]. Thus, the comparative accuracy of the impression techniques becomes a significant issue in consideration of passive fit. An inaccurate impression may result in prosthesis misfit, which can lead to further problems such as mechanical and/or biological complications [2]. Impression technique, type of impression material [3], splinting or non-splinting impression copings, type of splinting material, and number and angulation of implants [4] are the factors that affect the accuracy of impression. Two main implant impression techniques are used for transferring the intra-oral spatial relationship of the implants to the working cast. One impression technique is the direct open tray technique that uses a custom tray with windows exposing the impression copings. The other * Correspondence: [email protected]; [email protected] 1 Dental Biomaterials Department, Faculty of dentistry, Tanta University, Tanta, Egypt Full list of author information is available at the end of the article International Journal of Implant Dentistry © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 DOI 10.1186/s40729-018-0118-6

Cast accuracy obtained from different impression ...impression techniques at different implant angulations (in vitro study) Enas A. Elshenawy1*, Ahmed M. Alam-Eldein2 and Fadel A

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Page 1: Cast accuracy obtained from different impression ...impression techniques at different implant angulations (in vitro study) Enas A. Elshenawy1*, Ahmed M. Alam-Eldein2 and Fadel A

RESEARCH Open Access

Cast accuracy obtained from differentimpression techniques at different implantangulations (in vitro study)Enas A. Elshenawy1*, Ahmed M. Alam-Eldein2 and Fadel A. Abd Elfatah2

Abstract

Background: Angulated implants may result in inaccurate impressions, and the impression technique may affectthe accuracy of the definitive cast. This study was designed to compare the dimensional accuracy of casts obtainedfrom three impression techniques for three definitive lower casts with implants at different angulations.

Methods: Three Osseolink implants were placed in three reference models with different angles (parallel, 15° and30°). Impressions of each model were made with three techniques (n = 10 per group): indirect, unsplinted direct,and acrylic resin-splinted direct technique. Impressions were poured with type IV dental stone. Inter-implant distanceswere measured for casts using a coordinate measuring machine, and the deviations from the reference models (Δr)were calculated. Data were analyzed using one-way ANOVA followed by post hoc tests to detect significance betweengroups (α = 0.05).

Results: This study showed that the deviations in micrometers from the reference model were the least for acrylicresin-splinted direct technique (Δr1 = 49.96, Δr2 = 50.36) versus indirect (Δr1 = 93.8, Δr2 = 90.9) and unsplinted directtechniques ((Δr1 = 67.07, Δr2 = 68.66) in 30° angulated implant situation (p value < 0.0001* for both Δr1 and Δr2). In 15°angulated implants, both the acrylic resin-splinted direct (Δr1 = 44.64, Δr2 = 45.58) and unsplinted direct techniques(Δr1 = 47.39, Δr2 = 55.28) were more accurate than indirect technique (Δr1 = 64.8, Δr2 = 68.3) (p value < 0.0001* forboth Δr1 and Δr2). While in parallel condition, no difference was found between all three techniques (p value = 0.085,0.056 for Δr1 and Δr2, respectively).Conclusions: The impression technique affected the accuracy of definitive casts. The acrylic resin splinted directtechnique produced the most accurate casts, followed by direct unsplinted and indirect techniques. Furthermore,implant angulation affected the impression accuracy. When implant angulation increased from parallel implants to 30°,the forces of deformation increased, which resulted in increased distortion.

Keywords: Direct technique, Indirect technique, Internal connection implant, Splinting procedure

BackgroundPrecise working casts are essential to fabricate passivelyfitting implant prostheses. Accurate implant impressionsplay a significant role and serve as a starting point in theprocess of producing good working casts [1]. Thus, thecomparative accuracy of the impression techniquesbecomes a significant issue in consideration of passivefit. An inaccurate impression may result in prosthesis

misfit, which can lead to further problems such asmechanical and/or biological complications [2].Impression technique, type of impression material [3],

splinting or non-splinting impression copings, type ofsplinting material, and number and angulation ofimplants [4] are the factors that affect the accuracy ofimpression.Two main implant impression techniques are used for

transferring the intra-oral spatial relationship of theimplants to the working cast. One impression technique isthe direct open tray technique that uses a custom traywith windows exposing the impression copings. The other

* Correspondence: [email protected];[email protected] Biomaterials Department, Faculty of dentistry, Tanta University, Tanta,EgyptFull list of author information is available at the end of the article

International Journal ofImplant Dentistry

© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.

Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 DOI 10.1186/s40729-018-0118-6

Page 2: Cast accuracy obtained from different impression ...impression techniques at different implant angulations (in vitro study) Enas A. Elshenawy1*, Ahmed M. Alam-Eldein2 and Fadel A

impression technique is the indirect technique that usesclosed tray [5]. With the direct technique, both splintingand non-splinting of impression copings to improve theaccuracy of impressions have been advocated [6].In the open tray technique, the impression coping is

incorporated in the impression and is removed from themouth together with the set impression [1]. In theclosed tray technique, the impression copings areretained in the mouth when the set impression is re-moved, and then, these copings are unscrewed from themouth and connected to the implant analogs. Thiscoping-implant analog assembly is repositioned into itsrespective position within the impression [7].To ensure maximum accuracy, some authors empha-

sized the importance of splinting impression copingstogether intraorally before making an impression andsome authors sectioned the splint material leaving a thinspace and then rejoining with a minimal amount of thesame material to minimize polymerization shrinkage.However, inconsistent results have been obtained [8, 9].The implant impression can be at the abutment or

implant level. The implant level impression is preferredin the esthetic zones and reduces the number of treat-ment visits. However, it presents unique challenges tothe prosthodontist and errors can be introduced in manyways due to a rotation error that occurs when implantsare connected to impression copings and to dislodge-ment of the impression material during removal of theimpression tray from the mouth [10].The adoption of tilted implants for the rehabilitation

of both edentulous mandibles and maxillae has beenproposed in the recent years. In the mandible, tilting ofthe distal implants may prevent damage to the mandibu-lar nerve. Implants of conventional length can be placed,allowing engagement of as much cortical bone aspossible, thus increasing primary stability [11]. However,the lack of parallelism between implants may result inincreased distortion of impression material during re-moval from the mouth that may generate an inaccuratemodel [12–14].Several impression materials have been used for multi-

unit implant impression; the most commonly describedwere addition silicone and polyether impression mate-rials. This can be correlated to their improved accuracy[7]. Polyvinylsiloxanes show the smallest dimensionalchanges in comparison to the other elastomeric impres-sion materials since they do not produce a volatile by-product during polymerization [15, 16].This study was conducted to evaluate the effect of

impression techniques and implant angulations on theaccuracy of impressions in parallel and angulatedimplants in three mandibular models simulating clinicalsituations.Three null hypotheses were tested:

1. There is no significant difference in impressionaccuracy whether an indirect, direct unsplinted, ordirect acrylic resin splinted impression techniqueswere used.

2. There is no significant difference in impressionaccuracy whether implants had a 0°, 15°, or 30°angulation to a reference line perpendicular to thecast.

3. There is no significant interaction between theimpression technique and implant angulation.

MethodMaster model fabricationThree epoxy resin (Ramses medical products factory,Alex, Egypt) completely edentulous mandibular modelsrepresenting a clinical situation were used as definitivecasts. Each cast had three implants (OsseoLink USALLC. 4 mm × 9 mm, internal connection type) arrangedwith one implant at the midline and the other twoimplants at the premolar regions.

Cast (1) had all the three implants parallel to eachother and perpendicular to the plane of the cast.Cast (2) had implant at the midline perpendicular tothe plane of the cast and implants at the premolarregions angulated at 15° to a line drawn perpendicularto the occlusal plane.Cast (3) had implant at the midline perpendicular tothe plane of the cast and implants at the premolarregions angulated at 30° to a line drawn perpendicularto the occlusal plane.

Each definitive cast was held in a vertical milling ma-chine (Milling & Drilling machine, RF-Sakkary, Taiwan),and a protractor was used to align the cutting bur in theproper angulation by tilting the machine table.The implants were placed in each definitive cast with

a hand wrench and were numbered as follows: themiddle implant was number 1, the left premolar implantwas number 2, and the right premolar implant wasnumber 3; and this numbering was used throughout thestudy.

Custom tray fabricationPreparation of stone duplicate for each modelAfter the impression copings were connected to thedefinitive models, the space for impression material wascreated with baseplate wax (Cavex Setup Waxes,Haarlem, Holland). Stoppers were made on the molarregions to standardize the tray position.An impression was taken from each model, using

condensation silicone (Zetaplus, Zhermack SpA, Italy).Impressions were boxed and poured with type IV dentalstone (Elite® Stone, Zhermack GmbH Deutschland) in a

Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 Page 2 of 9

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vacuum device. The resulting three stone casts wereused to fabricate the custom trays.

Preparation of the master custom traySelf-cured acrylic resin (Acrostone cold cure special traymaterial, Cairo, Egypt) was used to make the mastercustom trays. There are two master trays for each cast:one closed tray for the indirect technique and one withthree windows for the direct unsplinted and acrylicresin-splinted technique.

Preparation of replicate custom traysDental flask was used for fabricating replicate traysfrom each master tray using the master custom trayand type IV dental stone to fabricate a two-part moldto make 30 custom trays for each cast, ten closedand twenty open trays.The trays were made of self-cured acrylic resin (Acros-

tone cold cure special tray material, Cairo, Egypt). Thetrays were perforated for added retention of the impres-sion material. Tray handles were made and attached tothe custom trays. The trays were stored at the roomtemperature for 24 h before impression taking.

Impression procedureThree different groups of implant level impression tech-niques were made (n = 10 per group) for each referencecast, a total of nine subgroups.Addition silicone impression material (Enthus PVS Im-

pression Material, Dharma Research, USA) with mediumconsistency was used for all impression procedures.The impression procedure was standardized as

follows:

1. A 1.5 kg metal block exerted a standardized pressureon each tray during the polymerization.

2. The copings were secured to the implants usingdedicated torque wrench calibrated at 10 Ncm.

3. Tray adhesive was painted on the trays beforemaking impressions.

4. The impression material was mixed using animpression gun.

5. The reference models were painted with separatingmedium before the impression procedures tosimulate oral condition.

In the indirect techniqueClosed tray impression copings remained on the definitivecast after removal of the impression. These impressioncopings were removed one at a time from the definitivecast and attached to an implant analog. The combinedimpression coping analog unit was inserted into theimpression by firmly pushing it into place to full depthand slightly rotating clockwise to feel for the anti-rotational resistance (Fig. 1 a, b).

In the direct unsplinted techniqueThe guide pins were loosened with a hex driver and re-moved, and the tray was separated with the impressioncopings locked in the impression. The guide pins wereplaced back into the impression copings from the top,while an implant analog was connected to the hex onthe bottom, and the guide pins were tightened with thedriver (Fig. 2).

In the direct acrylic resin-splinted techniqueThe direct impression copings were tied up with fourcomplete loops of dental floss (REACH® Mint WaxedFloss, Johnson and Johnson Personal Products) using aforceps. Autopolymerizing acrylic resin (Acrostone coldcure special tray material, Cairo, Egypt) was appliedaround the impression copings using an incrementalapplication technique till the surface of the transfercopings are fully covered with a layer about 2 mm inthickness. A silicone index (Zetaplus, Zhermack SpA,Italy) was made after the first splint for each cast tostandardize the amount of acrylic resin used and used asa reference for splinting.After 17 min, the splint was sectioned into three

pieces with a diamond disk. The impression copingswere then resplinted with same acrylic resin (Fig. 3 a, b).Another 17-min interval was allowed after additionalsplinting to reduce the effects of polymerizationshrinkage.

Fig. 1 a Attaching the analog to the coping using the screw driver. b Impression after insertion of the coping-analog till hearing the audible click

Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 Page 3 of 9

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Impressions were inspected and repeated when anyinaccuracy was found.

Cast production procedureAll the impressions were poured with type IV dentalstone (elite® stone, Zhermack GmbH Deutschland) usinga single prefabricated mold made with laboratorysilicone (Ramses medical products factory, Alex, Egypt).After setting of stone, the casts were separated from

the impressions. The three healing abutments weretightened to the implant analogs before the measuringprocedures. All casts were labeled and stored at roomtemperature for 24 h prior to measurements.

Measurement procedureA coordinate measuring machine (CMM) (MitutoyoCRYSTA-Apex S544, Japan) was used to evaluate thepositional accuracy of the samples with accuracy of0.0001 mm.The implant abutments were donated as seen in

(Fig. 4).The center of abutment 1 was considered as the refer-

ence point for all measurements. The planar surfacefrom this point was regarded as XY. Two imaginary XZ

lines were considered between the centers of the analogs1, 2 and 1, 3. The XZ planes were perpendicular to XYplane. Therefore, the center of analog 1 was laid on theorigin (0, 0, 0). CMM measured the coordinates of eachanalog with respect to the reference point (Fig. 5).The center of each implant abutments was located

using a CMM probe by touching eight points on thecircumference of the outer diameter of the implantabutments.Four points on the upper surface of each implant abut-

ment were measured to form a plane used to calculatethe vertical distances between implant abutments 1 and2, and 1 and 3 in the z-axis (Fig. 6).The distances (in micrometers) between the implant

centers with the reference point were calculated accor-ding to the following formula [9]:The distance from the reference point (r) =ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

x2 þ y2 þ z2p

.Absolute error (Δr) was calculated by comparing the

Euclidean distance between the analogs in the duplicatedcast with the distance in the definitive cast:

Absolute error Δr =ffiffiffiffiffiffiffiffi

x2mþp

y2mþz2m−

ffiffiffiffiffiffiffiffi

x2dþq

y2dþz2:d

where m =master and d = duplicated.

Fig. 2 Connecting the analog with the coping using the guide pin

Fig. 3 a The splint sectioned into three separate pieces. b Resplinting with acrylic resin

Fig. 4 Implant donations

Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 Page 4 of 9

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Each model has two Euclidean distances and namedΔr1 (absolute error between implant abutments 1 and 2)and Δr2 (absolute error between implant abutments 1and 3).

Statistical analysisMean and standard deviation were calculated for eachEuclidean distance. One-way ANOVA followed by posthoc tests was performed to detect significance betweengroups. Statistical analysis was performed using SPSSstatistics software for Windows. P values ≤ 0.05 wereconsidered to be statistically significant in all tests.Two-way ANOVA was used to evaluate the influence

of different impression techniques and implant angula-tions on the accuracy of impressions at a significancelevel of .05 (SPSS version 20, IBM).

ResultsThe mean and standard deviation values of each of thetwo Euclidean distances measured in micrometer (μm)for the nine study groups are presented in Table 1.

Comparing between study groupsEffect of impression technique on accuracy of finalimpression

In cast (1):One-way ANOVA revealed no statisticallysignificant differences in deformation between the threeimpression techniques (F = 2.694, 3.276; p value = .085,0.056 for Δr1 and Δr2, respectively) as shown inTable 2.

In cast (2): One-way ANOVA revealed statisticallysignificant differences in deformation between the threeimpression techniques (F = 30.03, 22.95; p value =< .0001* for Δr1 and Δr2, respectively) as shown inTable 3. Post hoc test showed that indirect techniquewas significantly (p < 0.05) less accurate than directunsplinted and direct splinted techniques which werenot significantly different from each other (p > 0.05) forΔr1 and Δr2.In cast (3): One-way ANOVA revealed statistically highsignificant differences in deformation between the threeimpression techniques (F = 85.65, 83.56 p value =< .0001* for Δr1 and Δr2, respectively) as shown inTable 4. Post hoc test showed that indirect techniquewas significantly (p < 0.05) less accurate than directunsplinted which was significantly (p < 0.05) less accur-ate than direct splinted techniques for Δr1 and Δr2.

Effect of implant angulation on accuracy of final impression

For (group 1) the indirect technique One-way ANOVArevealed statistically significant differences in deform-ation between cast 3(30৹), cast 2 (15৹), and cast 1(0৹)groups (F = 100.65, 71.39; p value = < .0001* for Δr1and Δr2, respectively). Post hoc test showed thatindirect technique was significantly (p < 0.05) lessaccurate in case of 30৹ angulated implants in cast 3than 15৹ angulated implants in cast 2, which wassignificantly (p < 0.05) less accurate than parallelimplants in cast 1 for Δr1 and Δr2.

Fig. 5 a Inter-implant distances in x- and y-axes. b Inter-implant distance in Z axis

Fig. 6 a Measuring the outer diameter. b Measuring the upper surface

Elshenawy et al. International Journal of Implant Dentistry (2018) 4:9 Page 5 of 9

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For (group 2) direct unsplinted technique One-wayANOVA revealed statistically significant differences indeformation between cast 3 (30৹), cast 2 (15৹), and cast 1(0৹) groups (F = 36.2, 9.33 p value = < 0.0001*, 0.0008* forΔr1 and Δr2, respectively). Post hoc test showed that thedistortion values of the duplicate casts obtained fromcast 3 (30৹) was significantly higher than distortionvalues for cast 1 (0৹) and cast 2 (15৹) (p < 0.05), whichwere not significantly different from each other (p >0.05) for Δr1 and Δr2.

For (group 3) direct splinted technique One-wayANOVA revealed no statistically significant differences indeformation (Δr1 or Δr2) between cast 3 (30৹), cast 2(15৹),and cast 1(0৹) groups (F = 3.14, 2.18; p value = .059, .132for Δr1 and Δr2, respectively).

Interaction between variablesA two-way ANOVA was performed to study the effectof impression technique and implant angulation on theaccuracy of duplicate casts. The data obtained in thisstudy reveals significant interaction between impressiontechnique and implant angulation (p = < .0001*) in thetwo Euclidean distances and that both variables affect

the implant impression accuracy as shown in Tables 5and 6.

DiscussionAn impression that precisely records the 3-dimensionalpositions of implants is essential to achieve a passivelyfitting prosthesis [1, 17]. Therefore, comparative accu-racy of impression techniques becomes an importantissue in consideration of passive fit [8].In this study, epoxy resin models were used as refer-

ence models because they have appropriate elasticmodulus for a bone analog material [18]. They were alsofound to have better stability than plaster models usedin other studies [19, 20].The models were selected to be with no undercuts

because undercuts need high removal forces, which canconfound the results. Therefore, removing them fromthe study favors the reliability of the findings [21].In the present study, the three implants were placed in

each reference model with different angulations to simu-late common clinical situations that may necessitateplacement of angulated implants in lower premolar re-gion. Furthermore, unlike most of previous studies, theimplants in this study were also tilted to the mesial side,which better represents clinical conditions [21].

Table 1 Mean and standard deviation values of each of the two Euclidean distances measured in micrometer (μm) for the studygroups

Groups Subgroups Mean ± SD

Δr1 Δr2

Group 1:Indirect techniquen = 10 per subgroup

Subgroup A: cast 1 48.11 ± 8.2 54.08 ± 7.2

Subgroup B: cast 2 64.8 ± 8.2 68.3 ± 8.6

Subgroup C: cast 3 93.8 ± 4.83 90.9 ± 4.2

Group 2:Direct unsplinted techniquen = 10 per subgroup

Subgroup A: cast 1 41.4 ± 9 50.7 ± 13.3

Subgroup B: cast 2 47.39 ± 5.51 55.28 ± 7.3

Subgroup C: cast 3 67.07 ± 5.7 68.66 ± 4.7

Group 3:Direct splinted techniquen = 10 per subgroup

Subgroup A: cast 1 40.3 ± 6.7 42.8 ± 8.5

Subgroup B: cast 2 44.64 ± 4.63 45.58 ± 3.4

Subgroup C: cast 3 49.96 ± 10.6 50.36 ± 10.3

Δr1 the absolute error between implant abutments 1 and 2, Δr2 the absolute error between implant abutments 1 and 3

Table 2 Comparison of impression techniques in cast 1 usingone-way ANOVA

Comparison of impression techniques incast 1 (parallel condition)

F p value

Group 1Indirect

Group 2D.unsplinted

Group 3D.splinted

Mean ± SD Mean ± SD Mean ± SD

Δr1 48.11 ± 8.2 41.4 ± 9 40.3 ± 6.7 2.694 .085

Δr2 54.08 ± 7.2 50.7 ± 13.3 42.8 ± 8.5 3.276 0.056

Δr1 the absolute error between implant abutments 1 and 2, Δr2 the absoluteerror between implant abutments 1 and 3

Table 3 Comparison of impression techniques in cast 2 usingone-way ANOVA

Comparison of impression techniques incast 2 (angulated 15৹)

F p value

Group 1Indirect

Group 2D.unsplinted

Group 3D.splinted

Mean ± SD Mean ± SD Mean ± SD

Δr1 64.8 ± 8.2 47.39 ± 5.51 44.64 ± 4.63 30.0341 < 0.0001*

Δr2 68.3 ± 8.6 55.28 ± 7.3 45.58 ± 3.4 22.9561 < 0.0001*

Δr1 the absolute error between implant abutments 1 and 2, Δr2 the absoluteerror between implant abutments 1 and 3

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In this study, impressions were made at implant levelbecause it allows for the selection of the most properabutments and is helpful in situations where angulationof the abutments is difficult to be determined intraorally[19, 22].The impression material used for this study was

polyvinylsiloxane as it exhibits accuracy and adequaterigidity [23]. Medium consistency was more advanta-geous than putty consistency because the implants usedcaused a higher level of stress to the impression copingsduring the impression procedure. Therefore, the use of amore elastic consistency is advantageous in evaluatingthe effect of splinting impression copings on impressionaccuracy [19]. In addition, the single-step techniqueallows the material to record finer details withoutslumping of the material in the tray, less time-consuming, and simple to perform [24].Custom trays were utilized because elastomeric mate-

rials are more accurate if used in 2 to 3-mm uniformthickness. All the custom trays were perforated to ensuregood retention with the trays [25]. Standardization ofcustom trays was done through modification ofreference models with spacer and making stoppers andthen making of the duplicate casts from the modifiedreference models [26].Self-cure acrylic resin was selected as a splinting

material in this study as it is easy to use and it doesnot require a dry environment [27]. Acrylic resinsplint was sectioned and resplinted after 17 min in

order to minimize any discrepancies due topolymerization shrinkage. Mojon et al [28] and otherstudies [19, 29–31] have stated that separation andreuniting of acrylic splint when done 17 min after thesetting reaction allows 80% reduction in the effects ofpolymerization shrinkage. A silicone index was madeto standardize the dimensions of the acrylic resinsplints for each specimen [19, 32].A prefabricated mold was used for pouring all impres-

sions to control the setting expansion and standardizethe amount of dental stone used [26]. All stone castswere stored at room temperature for 24 h prior tomeasurements to make sure that they have reached theiroptimal mechanical properties [19, 26, 33].Studies comparing the accuracy of implant impres-

sion techniques with methods such as micrometers,Vernier calipers, strain gauges, or measuringmicroscopes could merely carry out two-dimensionalmeasurements [5, 34–36]. However, when the mea-surements are two dimensional only, relevant infor-mation is lost. Therefore, CMM was used as themeasuring device in this study because it made three-dimensional evaluation of any distortion possible.When points from different implant casts have acommon reference within a coordinate system, the3D orientation of analogs can be recorded [37].The results show that there was no significant differ-

ence in accuracy between the impression techniquesused with parallel implants. The similar accuracy may bedue to removal of the custom tray along the same pathas the implant angulation. These results are in agree-ment with several studies showing no difference betweenthe three impression techniques [19, 20, 30, 38–40].While in the case of 15° angulated implants, direct

unsplinted technique and direct acrylic resin-splintedtechnique exhibited more accuracy compared to indirecttechnique. This was in agreement with some studies thatfound that direct impression technique whether splintedor not is significantly more accurate than indirecttechnique when angulation of implants increased up to15° [33, 39].

Table 4 Comparison of impression techniques in cast 3 usingone-way ANOVA

Comparison of impression techniquesin cast 3 (angulated 30৹)

F p value

Group 1Indirect

Group 2D.unsplinted

Group 3D.splinted

Mean ± SD Mean ± SD Mean ± SD

Δr1 93.8 ± 4.83 67.07 ± 5.7 49.96 ± 10.6 85.6521 < 0.0001*

Δr2 90.9 ± 4.2 68.66 ± 4.7 50.36 ± 10.3 83.5686 < 0.0001*

Δr1 the absolute error between implant abutments 1 and 2, Δr2 the absoluteerror between implant abutments 1 and 3

Table 5 Effect of implant angulation and impression techniqueon impressions by two-way ANOVA for Δr1Source of variation DF Mean square F Sig.

Angulation 2 5668.941 104.382 .000*

Imp.tech. 2 4562.156 84.003 .000*

Angulation * imp.tech. 4 860.574 15.846 .000*

Error 81 54.309

Total 90

Corrected total 89

DF degree of freedom*Significant (p < 0.05)

Table 6 Effect of implant angulation and impression techniqueon impressions by two-way ANOVA for Δr2Source of variation DF Mean square F Sig.

Angulation 2 3356.616 50.873 .000*

Imp.tech. 2 4557.783 69.077 .000*

Angulation * imp.tech. 4 581.127 8.808 .000*

Error 81 65.981

Total 90

Corrected total 89

DF degree of freedom*Significant (p < 0.05)

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Furthermore, in the case of 30° angulated implants, thedirect acrylic resin-splinted technique was significantlymore accurate than the direct unsplinted technique,which was significantly more accurate than the indirecttechnique. This finding is in agreement with severalstudies, which reported the superiority of the splintedtechnique over the non-splinted technique for makingan impression of angulated internal connection implants[6, 29, 31, 39, 41].Regarding implant angulation, the results of this

study found that increasing implant angulation to 15°or 30° affected the accuracy of indirect impressiontechnique. While in the direct unsplinted technique,no difference in accuracy was found between parallelcondition and 15° angulated condition. The increaseddisplacement of impression material and the difficultremoval of the impression tray in case of angulatedimplant were believed to be the source of error inthe indirect impression technique. In the directtechnique, the impression coping remains in the im-pression, which reduces the effect of implant angula-tion and the impression material deformation uponremoval from mouth.These findings agreed with Lee et al. [33] and Carr

et al. [42] who found significant difference in accur-acy of indirect technique with angulated implants,while no difference in accuracy of direct technique upto 10° and 15° angulations, respectively. Conrad et al.[20] found that angulation of implants up to 15° didnot affect the accuracy of both indirect and directtechniques.In this study, increasing the angulation between

implants to 30° affected the accuracy of directunsplinted technique while it did not affect the accu-racy of direct splinted technique significantly. This isin agreement with Tsagkalidis et al. [39] and Martínez-Ruset al. [31]. This may be because splinting the impres-sion copings using a rigid material prevented indivi-dual coping movement during the impression makingprocedure [1].This study showed that significant interaction

existed between impression technique and implantangulations and that both affected implant impressionaccuracy. As implant angulations increase, distortionin the experimental cast increases. This can beexplained with increased material deformation uponimpression removal. These results find support insome other studies [39, 43].According to the recorded data, the null hypothesis

was partially rejected because the accuracy of theimpression techniques was only different in angulatedimplant conditions and there was an interaction betweenimpression technique and implant angulations and thatboth affect impression accuracy.

ConclusionsThe accuracy of definitive casts was affected by the im-pression technique only in angulated implant conditionswhere direct splinted technique provided the mostaccurate position transfer. In parallel implant situation,the three techniques were similar.When implant angulation increases, the forces of

deformation increase which requires an impression tech-nique that allows precise inter-implant relationship. Theindirect technique showed the highest distortion valueswhen angulated implants were used followed by direct-unsplinted technique then direct acrylic resin-splintedtechnique.

Authors’ contributionsEE carried out samples preparation, measurements, data collection, anddrafting the manuscript. AM participated in the design of experiment andperformed the statistical analysis and interpretation of data, and FAparticipated in conception, design, and revising manuscript critically forimportant intellectual content. All authors read and approved the finalmanuscript.

Competing interestsEnas A. Elshenawy, Ahmed M. Alam-Eldein, and Fadel A. Abd Elfatah declarethat they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Author details1Dental Biomaterials Department, Faculty of dentistry, Tanta University, Tanta,Egypt. 2Prosthodontic Department, Faculty of dentistry, Tanta University,Tanta, Egypt.

Received: 25 October 2017 Accepted: 10 January 2018

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